A conductive molding is used widely as an electrode for a cell and a separator for a fuel cell.
The fuel cell taking out an electric energy by supplying fuel (reducing agent) and oxygen (oxidizing agent) successively from outside is excellent in its power generation efficiency, capable of generating electricity without using fossil fuel, and discharges only water as a discharge gas. Therefore development of the fuel cell is being made as an energy source kind to the global environment. Attention is particularly paid to a solid polyelectrolytic fuel cell which is operated at a low temperature of about 80° C. and yet capable of discharging a comparatively high current and thereby serves as a power source for household appliances and electric cars.
In the solid polyelectrolytic fuel cell, an anode is disposed at one side of a polymeric film, and a cathode is disposed at the other side thereof to form a membrane-electrode-joined assembly. An anode-side duct substrate supplying hydrogen serving as fuel is disposed at one side of this assembly and a cathode-side duct substrate supplying oxygen is disposed at the other side thereof to form a unit cell. Unit cells are layered one upon another through separators. Alternatively, the separators are layered one upon another by using them as a flow path substrate.
As the separator serving as the flow path substrate, the separator on which a flow path is formed by cutting a graphite block is known, and the separator obtained by molding an expanded graphite sheet at a high pressure is also known. As conventional separators, made of a resin material, for use in the fuel cell, the separator (international laid-open No. WO 097/02612) for the fuel cell in which the expanded graphite powder, whose particle diameter is specified, is dispersed in thermoplastic resin or thermosetting resin is known. The separator (Japanese Patent Laid-Open No. H 11-354135) for the fuel cell in which a carbon material such as expanded graphite powder is dispersed in phenol resin cured by ring opening polymerization is also known.
As a method of injection-molding ultra-high-molecular-weight polyethylene, the method of injecting it into the cavity of a die at a shearing speed of not less than 50×103/s and then compressing the volume of the cavity to not more than 2.0 folds of the amount of injection-molded resin (Japanese Patent Application Laid-Open No. S 51-81861) is known.
However, the conventional separators for use in the fuel cell using the resin material has a problem that they do not have sufficient conductivity, unpermeability to gases, resistance to acids, resistance to corrosion, and mechanical strength.
When the resin material is used, a large amount of a compounding agent is added to the resin material to improve conductivity. As a result, the melting viscosity of the resin composition rises outstandingly. Therefore, frequently it is difficult to mold the resin composition, the molding method is limited, and only a molding having a simple configuration is obtained. In the case of the conductive resin molding such as the separator serving as the flow path substrate, it is difficult to accomplish integral molding of a flow path. Thus it is necessary to form a complicated duct groove by post-processing such as a cutting process. Therefore the conventional conductive resin molding has a problem that the production yield of a material lowers and the machining cost increases.
Even if a small conductive resin molding can be formed from the conventional resinous material, the obtained conductive resin molding cannot be used as the separator for use in the fuel cell which withstands practical use. For example, the obtained conductive resin molding has a low denseness, is incapable of satisfying required characteristics such as gas unpermeability, electric resistance, and generates a warp. The conductive resin molding whose matrix consists of thermo setting resin is post-cured after the resin composition thereof is molded. At that time, dimensional contraction or deformation occurs. Thus it is difficult to obtain a separator having a high dimensional accuracy. In the separator for use in the fuel cell, a plurality of unit cells are laminated one upon another. Therefore if the flatness degree thereof is low, the separator cannot be used.
A problem has arisen that to make the fuel cell thin and compact, a more complicated flow path groove is demanded, but the conventional cutting process has difficulty in industrializing the formation of the complicated flow path groove in terms of cost. Another problem is that the conventional conductive resin material has a deformation such as a warp and a twist, which makes it difficult to hold dimensional accuracy.
The present invention has been made to cope with the problems. Therefore it is an object of the present invention to provide a conductive resin molding and a method of producing the conductive resin molding capable of suppressing the lowering of the production yield of a separator made of the conductive resin molding and an increase of the processing cost which are the most important problem in the spread of a fuel cell.